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Since all of these fast lenses have severe vignetting wide open, I wonder how they measure T stops. Do they measure average across the whole frame, or only in the center, or somewhere in between these two extremes?

It looks like they measure it across the frame, and they don't mention vignette compensation:

That would explain why a slow IS zoom like the 24-70 F/4 IS has T=4 whereas all fast lenses have T stop significantly above their largest aperture. In other words, this T stop number is a poor substitute for their vignetting number and as such should be considered most useless.